Twitter started out as a good idea with lots of users. These attracted
investors, who have been willing to fund it lavishly. But it had no way of
making money. This is not unusual for internet companies. Google and
Facebook also started life as cool products that people loved, and thought
only later about how to create a business model.

That moment came three years ago, when Twitter introduced advertising on the
site. But instead of plastering it with marketing, Twitter incorporated
adverts inside its system with “promoted tweets” in which advertisers can
pay for